1. Technical Field
The present invention relates to voltage controlled oscillators (VCO) in general, and more particularly to a VCO with tuneable active inductors for tuning a frequency of oscillation,
2. Discussion
Maturing silicon bipolar and Gallium Arsenide (GaAs) hetero-junction bipolar technologies have made microwave monolithic integrated circuits (MMIC) feasible. A common goal for any monolithic circuit is to use design and fabrication techniques to reduce chip size, increase ease of implementation, and reduce costs in high volume production. These design and fabrication techniques should not compromise performance, and if possible, should improve performance.
Microwave frequency voltage controlled oscillators are used widely in High Definition Television (HDTV), cellular phone systems, optical/analog phase-locked loop communication systems, etc. Conventional VCO designs use a series inductance in a feedback loop of a transistor and a fixed capacitance in series with an emitter of the transistor to generate a frequency of oscillation. For monolithic designs, the series inductance is typically a spiral inductor. Fabrication of the spiral inductor on a monolithic chip requires careful modeling, a complicated fabrication process, and large surface areas on a semi-insulating semiconductor substrate such as GaAs.
The surface area required for an inductor increases as the inductance value increases. Since microwave applications typically require inductors with relatively high inductance, the surface area of these inductors adversely affects chip size.
One proposed fabrication solution is to use a conventional multi-vibrator chip in an analog phase-locked loop (PLL) system. While the PLL system does not require spiral inductors and is very compact in size, the PLL system exceeds acceptable phase noise due to its inherently low Q.